1. Field of the Invention
The invention relates in general to wireless communication, and more particularly, to a calibration method and apparatus for intermodulation distortion in wireless communication.
2. Description of the Related Art
A wireless communication system, a radio-frequency (RF) front-end circuit in particular, usually contains non-ideality of non-linearity. When an input end of a wireless communication system receives external interferences, and especially when interference signals simultaneously having two or more different frequencies, a crossover component is incurred in the two interference signals due to the non-linearity of the system. If the frequency of the crossover component is similar to that of a target signal, the target signal becomes interfered to lead to demodulation complications, as well as a reduced sensitivity of the system. For example, assume frequencies of input signals are (fRF+f1), (fRF+f2) and (fRF+f3). The frequencies f1, f2 and f3 are target frequencies; the frequency fRF is the RF carrier frequency. The frequencies of second-order crossover components can be a sum of or a difference between every two target frequencies, e.g., (f1+f2), (f1−f2), (f1+f3) or (f1−f3); the frequencies of third-order components can be a combination of the three target frequencies, e.g., (f1+f2+f3) or (f1+f2−f3).
For a communication system in a superheterodyne structure based on an intermediate frequency (IF), interferences of third-order crossover components are usually more obvious. A receiver in a superheterodyne structure first down-converts an RF signal to an IF signal and then further to a baseband signal. Provided that the frequency position of the IF is appropriately selected for cooperating with a filter, second-order crossover components generally do not cause damages.
Moreover, for a communication system in a homodyne structure (or referred to as a direct-conversion structure) that does not involve IF signals or in a low IF structure based on IF signals in extremely low frequencies, interferences of second-order crossover components are more severe. In a homodyne structure, a receiver directly down-converts an RF signal to a baseband signal, and a transmitter directly up-converts a baseband signal to an RF signal. A homodyne structure is more cost-competitive as IF and associated circuits are eliminated, and thus prevails in certain cost-effective 3C communication products. However, second-order crossover components are prone to occur near target frequencies since only one up-conversion/down-conversion process is performed in the modulation. Therefore, in order to enhance signal sensitivity in a wireless communication system, calibration is needed for second-order intermodulation distortion to reduce the second-order crossover components generated during the modulation.
FIG. 1 shows a direct-conversion receiver 10 supporting second-order intermodulation distortion calibration. The receiver 10 may be implemented by an integrated circuit. A low-noise amplifier (LNA) 14 amplifies an RF signal received by an antenna 12. A local oscillator 20 provides local oscillation signals in a 90-degree phase difference to mixers 16 and 18, respectively, to down-convert the RF signal to generate a baseband signal. A receiver core 22 processes the baseband signal, e.g., by demodulation, to obtain information carried in the baseband signal.
A calibrator 26 performs second-order intermodulation distortion calibration during a final test after the receiver 10 is packaged. A test machine provides two known RF test signals in different frequencies (fRF+ft1) and (fRF+ft2) to an RF input end of the low-noise amplifier 14. The mixers 16 and 18 produce expectable target signals (having frequencies ft1 and ft2) and possible second-order crossover components (having frequencies ft1−ft2 and ft1+ft2), as shown in FIG. 1. The calibrator 26 adjusts the mixers 16 and 18 according to signal strengths of the second-order crossover components to minimize the signal strengths of the second-order crossover components.
A main reason to incur the second-order crossover components is that, in two ideal and completely matching different signal receiving paths, mismatching however is resulted from inevitable differences due to variations in a fabricating process or a circuit layout of an integrated circuit. FIG. 2 shows a mixer 30 operable in cooperation with second-order intermodulation calibration. The mixer 30 receives RF signals via two differential input ends RF+ and RF−, receives local oscillation signals via two other differential input ends LO+ and LO−, and outputs output signals VOUT via two different output ends. Resistors RP and RN serve as output loads of the mixer 30. A calibrator may detect the signal strength of second-order crossover components in the output signals VOUT to adjust a calibration resistor RCALI, so as to fine-tune the matching between two differential signal paths including the resistors RP and RN.
However, in the above calibration method adopted in the final test, a desired calibration result can only be obtained after an extremely time-consuming process. In other words, substantial associated test costs are involved such that product competitiveness is lowered.